Deep-hardening silicon steel



Patented Aug. 17, 1943 2,326,838 DEEP-HARDENING SILICON STEEL WalterCrafts, Niagara Falls, N. Y., assignor to Electro Metallurgical Company,a corporation of West Virginia No Drawing. Application March 2, 1940,Serial No. 321,915

Claims. (01. 14831) This invention relates to steels, and morespecifically to steels the hardness of which may be increased by rapidcooling from temperatures within or above the critical range. For thesake of simplicity of presentation, the application of the inventionwill be described herein principally as it may be applied to carbonsteels containing between 0.2% and 1% carbon, up to about 2% manganese,and up to about 2% sili con; but as the description proceeds it will beevident that the invention may be applied to a wide variety ofhardenable steels.

When hardening steels by rapid cooling from high temperatures, it isoften desired to produce a deep or thick zone of hardened materialrather than a thin or shallow hardened case. The depth to which a pieceof steel will harden, at a given rate of heat extraction, is difierentin difierent steels. The property of the steel which involves thisrelative susceptibility to mass efiect in hardening seems to beinherent, and for convenience it will be termed hereindeep-hardenability.

An accepted, convenient measure of deephardenability is afforded by theJominy" test, described in detail in A hardenability test forcarburizing steel by W. E. Jominy and A. L. Boegehold, Trans. Am. Soc.for Metals, vol. 26, p. 5'74 (1938). To summarize briefly, the test ismade on a small bar of steel of standardized shape and dimensions, andcomprises heating the entire bar to the desired hardening temperature,quickly extracting heat through one end face of the bar, grinding ofisuperficial scale and decarburized skin and producing a flat surfacesuitable for a hardness test, and measuring the Rockwell C hardnessalong the length of the bar. The distance from the hardened end at whichthe hardness becomes less than Rockwell C 50 is referred to herein asthe Jominy depth. If the same hardening temperature and coolingconditions be used for a series of steels, the relative depths of thehardened zones indicate the relative deep-hardenability of the steels ofthat series, each to the others.

An object of this invention is to improve the deep-hardenability ofhardenable steels generally, and of plain carbon and low alloy steels(less than 5% in the aggregate of steel-strengthening elements forexample chromium, tungsten, molybdenum and nickel) especially.

Further objects -are to provide deep-hardenable steels of novelcompositions, and deephardened articles made therefrom.

' The deep-hardenability of steel may be enhanced in some degree byadding in suitable percentages any one of the elements which can betaken into solid solution in steel at high temperatures, for instance,boron, silicon, molybdenum, tungsten, chromium, or nickel. Thesehardening elements diifer widely in the minimum percentage required toimpart a useful degree of deep-hardenability, and in the maximum degreeof deep-hardenability obtainable while maintaining other necessaryproperties of the steel.

1 have observed that combinations of certain elements may be used toimpart a greatly enhanced deep-hardenability without substantialsacrifice of other desirable properties of the steel. The presentinvention is based on these observations.

More specifically, the invention comprises the addition to a hardenablesilicon steel of more than two of the special elements, aluminum,magnesium, calcium, barium, strontium, titanium, zirconium, andvanadium, in a suitable percentage greater than that required for grainrefinement.

The silicon content of the steel may suitably be between 0.1% and 2%. Ifthe silicon content is above 0.35%, the total proportion of said specialelements may be any proportion substantially greater than that requiredfor grain refinement, up to say 1%, or somewhat higher. The percentagerequired for grain refinement will depend on the kind of steel, thesteelmaking conditions, and the kind and number of deoxidizing andgrainrefining elements used, and may be determined empirically by themethods now in use by metallurgists and steelmakers. 'In most instances,at least 0.03%, and preferably at least 0.1%, of said special elementsare added, and it will not ordinarily be necessary to add more than0.25%.

If the silicon content does not exceed 0.35%, somewhat more of thespecial elements must be added to achieve a useful enhancement ofhardenability. In general, the aggregate percentage of residual siliconplus added special elements should exceed 0.35%, and preferably exceeds0.65%.

When the above-described percentages of silicon and special elements areemployed, a medium manganese oil hardening type of steel such as the S.A. E. 1345 type may be hardened to 50 Rockwell C or higher throughout acircular section oneinch, or even more, in diameter.

For each element and each combination of elements there appears to be anoptimum percentage which imparts a maximum depth of hardenability and afrequent result of an increasein percentage beyond the optimum is adecrease; of deep-hardenability below that imparted by the optimum. Forreasons of economy, or to obtain a steel having a certain desiredcombination of physical properties, it will often be desired to addeither less or more of the elements than will impart a maximum depth ofhardening. Hence, the invention is not limited to the use of the optimumpercentages.

In most cases it will be preferred to use alkaline earth metals only incombination with other elements of those listed above, inasmuch as theirbeneficial effects are considerably greater when they are used in suchcombinations than when they are used alone.

Typical effects of representative combinations used according to thisinvention are indicated in Table A, which contains figures derived fromactual test data. The relative Jominy hardenability depths are given, inhundredths of an inch, of steels containing besides iron the perandmanganese are by analysis, the percentage of silicon is by analysisexcept where indicated to be nominal, and the percentages of theremaining element are the percentages added to the steel just beforecasting. In Table 0, the yield point (Y. P.) and tensile strength (T.S.) of the steels of Table B are given in thousands of pounds per squareinch; percent El. designates percentage elongation in a two inch initialgage length, and percent R. A. designates percentage reduction in area,upon fracture of the 0.505 inch diameter standard (A. S. T. M.) tensiletest specimens. Under Izod are given the Izod impact test results, infoot-pounds, using a standard specimen one-centimeter square withstandard V notch one mm. deep. The Jominy hardness test depth is thatactually measured, in hundredths of an inch, to Rockwell C 50, the samehardening temperature and cooling conditions having been used in allhardness tests.

Table B centage of silicon and special elements indicated. 0 The samehardening temperature and cooling steels (rest 1mm) conditions were usedin all tests.

Steel Pert P61; P61; Per; Per Pert cen cen cen cen cent cen Table A 0 MnI Si Al 2: v

Percentage addition to Jominy hardness test, depth in 4 0.51 1.680.6002) 0.535 0.035 0.035 steels containing 0.25% hundredths of an inchto Rock- 5 0.61 1.61 0.50m) 0.035 0. 535 0. 035 to 1% Si, about 1.0%Well 0 50, corrected to 0.45% C 8 0.48 1.70 0. 75(1t) 0.105 0.105 0.105Mn, 0.45% to 0.55% C, by factor: rest Fe :l=0.0l% Q==l=0.012 inch(n)=Nominal. Percent Percent Percent 2 Table C Zr V steel steel steelsteel Properties of steels of Table B 0. 035 0. 032 0. 23 $7 0.105 0.10. 4 6 l N 0.535 0. 035 0. 035 e1 swel 0 P Per 15:33:51 0.035 0.5350.035 67 i (I) Y. P T. S cent cent Izod test El. ILA. depth 1 Notdetermined.

207.0 7.0 25.4 7.5 67 The fact that considerable deep-hardenability213.3 0.5 32.1 7.5 74 is imparted by any of a large number ofcombinations of elements is an important one, because the latitude inchoice of elements affords an opportunity to control the cleanness,grain size, tensile strength, and toughness of the steel throughselection of appropriate combinations. In general, the deep-hardeningsteel of best quality for most purposes will be obtained by the additionof more than two of the special elements described above, within therange of percentages suggested herein. This generalization holds truenot only for the steels of the S. A. E., 1345 type (medium manganese oilhardening) chosen for purposes of exemplification in Table A, but forlow-alloy steels of all types, including those containing one or more ofthe elements chromium, tungsten, molybdenum, nickel, phosphorus, sulfur,etc. However, it is not to be understood that all properties necessarilyimprove in a consistent procession as the number and percentage of addedelements increases. It must be 'borne in mind that the relativeeffectiveness of different elements is not the same, and also thatcertain combinations of elements are better in some respects than othercombinations comprising the same number of elements The high qualityattainable in steels according to this invention is indicated in TablesB and C by test data of the physical properties of severalrepresentative steels after forging, quenching from 850 C., and drawingat 400 C. for one hour. In Table B, the indicated percentages of carbonThe complex deoxidizing agents described in the patents numbered2,221,781; 2,221,782; 2,221,783; 2,221,784; and 2,269,407, granted onjoint applications by James H. Critchett and Walter Crafts, areadmirably suited for use in accordance with the present invention,provided they are used in a proportion substantially greater than thatrequired for grain refinement.

I am aware that Some slight increase in deephardenability has beenobserved to accompany the grain coarsening brought about by adding, to alow silicon steel, aluminum in excess of that required for grainrefinement. Such effect should not be confused with the considerableincrease in deep-hardenability obtained according to this invention inthe high silicon alloys.

The advantages of the invention may be exploited in any of several ways.For instance, the cheaper steels among those described above may be usedinstead of more expensive, more highly alloyed steels heretofore used toobtain the desired strength. Or, present high strength steels may beeven further strengthened by applying the principles of the invention,either by deeper hardening to a lower average hardness or by deeperhardening to the same or even higher hardness. Among the various steelsdescribed, there is a wide range of choice in respect to such factors ascost, grain size control, ductility, strength, toughness, and types ofinclusions. Thus, the invention is capable of a Wide field of asaaaseapplication, which will be apparent to metallurgists and steelmakers.Therefore, although numerous specific examples have been given herein toillustrate the principles of the invention, it will be understood thatsuch examples ar merely illustrative and do not restrict the inventionbeyond the requirements f the claims and the state of the art.

I claim: 1. A quench-hardened article composed.of a deep-hardenablesteel comprising 0.1% to 1% silicon; between 0.2% and 1% carbon;manganese in a percentage up to about 2%; vanadium in a percentagebetween 0.03% and 1%; and between 0.03% and 0.25% of each of the grainrefining elements aluminum, zirconium, and titanium; the

aggregate percentage of silicon and said grain refining elements beingat least 0.65%; remainder iron, such steel having the property of beingquench-hardenable to upwards of 50 Rockwell C to a depth at least 0.3inch below the surface throughout a one inch diameter circular section.

2. A quench-hardened article composed of a deep-hardenable steelcomprising 0.1% to 2% silicon; between 0.2% and 1% carbon; manganese ina substantial percentage up to about 2%; 0.03% to 0.5% vanadium; 0.03%to 0.5% aluminum; 0.03% to 0.5% zirconium; 0.03% to 0.5% titanium;remainder iron and incidental impurities; the aggregate percentage ofsilicon, vanadium, aluminum, zirconium and titanium being at least 0.35%and the aggregate percentage of aluminum, vanadium, zirconium andtitanium being not over 1%; such steel having the property of beingquench-hardenable to upwards of 50 Rock-' well C to a depth at least 0.3inch below the surface throughout a one inch diameter circular section.

3. A quench-hardened. article, other than a nitride case-hardenedarticle, composed of a deep-hardenable steel comprising 0.1% to 2%silicon; 0.2% to 1% carbon; manganese in a substantial percentage up toabout 2%; at least 0.03% but less than 0.5% aluminum; at least 0.03% ofeach of at least two of the grain refining elements titanium, vanadiumand zirconium, the aggregate remainder iron and incidental impurities;the eggreg'ate percentage of silicon, aluminum, and

' said grain refining elements being at least 0.65%;

such steel having the property of being quench hardenable to upwards ofRockwell C to a depth at least 0.3 inch below the surface throughout aone inch diameter circular section.

4. An article as claimed in claim 3, further characterized in that itcontains at least one of the alkaline earth metals magnesium, calcium,barium and strontium, such metals being added in an aggregate amount ofbetween 0.03% and 0.5%.

5. A quench-hardened article as claimed in claim 3, other than a nitridecase-hardened article, composed of a deep-hardenable steel having theproperty of being quench hardenable to upwards of 50 Rockwell C to adepth at least 0.3 inch below the surface throughout a one inch diametercircular section, and further characterized in that said steel comprises0.1% to 2% silicon, 0.2% .to 1% carbon, manganese in a substantialpercentage up to about 2%, at least 0.03% but less than 0.5% aluminum,0.03% to 0.97% zirconium, and 0.03% to 0.97% vanadium, the remainderiron and incidental impurities; the aggregate percentage of zirconiumand vanadium being not over 1% and the aggregate percentage of silicon,aluminum, zirconium, and vanadium being at least 0.65%,

WALTER CRAFTS.

